After 1. d4 d5 2. Nf3 Stockfish gives the best move as 2...e6, entering a QGD position.
Why does Black voluntarily block the LSB instead of develop it first with 2...Bf5?
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First of all, don't attribute too much weight to engine evaluations that early in the game. Rather look at the established theory, and what you want to achieve.
For example, after 1. d4 d5 2. Nf3, I am far from convinced that 2. ...e6 is the best move. I am sure that the most common move is 2. ...Nf6, which keeps your options open. You could for example play the Slav defence (...c6 after White goes c4) and develop your light-squared bishop later. Also in many lines of the London System you can still develop that bishop before going ...e6.
That said, I have indeed played 2. ...Bf5 on a few occasions in serious tournament games. One line I dislike is 3. c4 e6 (c6 is a not so great Slav line) 4. Nc3 Nf6 (Nc6 is possible, but has its own issues) 5. cxd5 exd5 6. Qb3 Nc6 7. Bg5 and the double attack on d5 and b7 basically forces 7. ...Na5 8. Qa4+ c6 9. Bxf6 gxf6 which isn't horrible for Black, but probably slightly worse than what you can get by defending a standard QGD, especially with White committed to an early Nf3 which rules out some critical lines.